The present invention is related to positioning apparatus and, more particularly, to positioning apparatus including at least one table linearly movable along an axis and supported by air bearing pads.
Workpiece positioning tables and, in particular, massive workpiece positioning tables servocontrolled for motion along a linear axis, are commonly supported by air bearing pads and are guided in the linear direction by air guide bearings facing opposing sides of a straightedge. In one type of such workpiece positioning tables, the air bearing pads are supported on a film of air above a ground and lapped granite surface. The flatness of the surface permits a very thin layer of air, typically a few 10,000th of an inch, to support a table having a substantial weight.
However, even precisely ground and lapped granite has certain unavoidable surface irregularities therein which require a greater average height of the air bearing pads above the granite surface than is otherwise desirable for accuracy of positioning.